The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a local coil which may be used to provide localized reception of the NMR signals produced in a whole body NMR scanner system or the like.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.1, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence, the magnitude of the net transverse magnetic moment M.sub.1 depends primarily on the length of time and magnitude of the applied excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.O, is determined by the magnitude of the transverse magnetic moment M.sub.1. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.o e t/T*.sub.2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more excitation pulses (B.sub.1) of varying magnitude and duration. Such excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.1 over a range of resonant frequencies. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is subjected to a sequence of NMR measurement cycles which vary according to the particular localization method being used. The received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.O, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the NMR signals can be identified.
NMR has rapidly developed into an imaging modality which is utilized to obtain tomographic, projection and volumetric images of anatomical features of live human subjects. Such images depict the nuclear-spin distribution (typically protons associated with water and fat), modified by specific NMR properties of tissues, such as spin lattice (T.sub.1), and spin-spin (T.sub.2) relaxation time constants. They are of medical diagnostic value because they depict anatomy and allow tissue characterization.
The NMR scanners which implement these NMR techniques are constructed in a variety of sizes. Small, specially designed machines, are employed to examine laboratory animals or to provide images of specific parts of the human body. On the other hand, "whole body" NMR scanners are sufficiently large to receive an entire human body and produce an image of any portion thereof.
There are a number of techniques employed to produce the excitation field (B.sub.1) and receive the NMR signal. The simplest and most commonly used structure is a single coil and associated tuning capacitor which serves to both produce the excitation signal and receive the resulting NMR signal. This resonant circuit is electronically switched between the excitation circuitry and the receiver circuitry during each measurement cycle. Such structures are quite commonly employed in both small NMR scanners and whole body NMR scanners.
It is also quite common to employ separate excitation coils and receiver coils. While such NMR scanners require additional hardware, the complexities of electronic switching associated with the use of a single coil are eliminated and specially designed coils may be employed for the excitation and receiver functions. For example, in whole body NMR scanners it is desirable to produce a circularly polarized excitation field (B.sub.1) by using two pairs of coils which are orthogonally oriented, and which are driven with separate excitation signals that are phase shifted 90.degree. with respect to each other. Such an excitation field is not possible with a single coil.
It is very difficult to construct a large coil which has both a uniform and high sensitivity to the NMR signal produced in a whole body NMR scanner. As a result, another commonly used technique is to employ "local" coils to either generate the excitation signal (B.sub.1), receive the resulting NMR signal, or both generate and receive. Such local coils are relatively small and are constructed to produce the desired field or receive the NMR signal from a localized portion of the patient. For example, different local coils may be employed for imaging the head and neck, legs and arms, or various internal organs. When used as a receiver, the local coil should be designed to provide a relatively uniform sensitivity to the NMR signals produced by the spin throughout the region of interest.
Recently, a novel resonator structure, referred to in the art as a "loop-gap" resonator, has been applied as a local NMR coil. As indicated in U.S. Pat. Nos. 4,435,680; 4,446,429; 4,480,239 and 4,504,577, the loop-gap resonator may take a wide variety of shapes. In all cases, however, a lumped circuit resonator is formed in which a conductive loop is the inductive element and one or more gaps are formed in this loop to form a capacitive element. While the loop-gap resonator has many desirable characteristics normally associated with lumped circuit resonators, it also has some characteristics normally associated with cavity resonators. Most notable of these is the much higher quality factor, or "Q", of the loop-gap resonator over the traditional lumped circuit resonators. When applied to NMR scanners, this higher Q translates into higher resolution images.
Local receiver coils may be positioned in any orientation with respect to the generated excitation field (B.sub.1) in order to receive the desired NMR signals. To prevent damage to the local coil (or receiver) and to prevent distortion of the excitation field (B.sub.1), the local coil is designed to be intrinsically isolated from the uniform excitation field. This is accomplished by employing pairs of loop-gap resonators and connecting them together in such manner that they have little sensitivity to a uniform excitation field. Such local coils are described in co-pending U.S. patent application Ser. No. 731,923, filed on May 8, 1985, and entitled "Loop-gap Resonator for Localized NMR Imaging." To couple the NMR signal from such local coils, however, a "bifilar" coupling loop is commonly employed. Such a coupling loop connects to the transmission line which leads to the receiver input and it extends into the interior of the loop-gap resonator where it couples to the sensed NMR signal. Such coupling loops are expensive to manufacture because they must be precisely shaped and positioned in order to preserve the intrinsic isolation characteristic.